1K ZX Chess is a 1982 chess-playing computer program (while missing three rules) for the unexpanded Sinclair ZX81.
1K ZX Chess uses only 672 bytes of RAM but implements most chess rules (castling, promotion, and en passant capture are missing), and a computer opponent. It was the smallest implementation of chess on any computer until its record was broken in January 2015 by the smallest completely new PC compatible BootChess, although apparently artificial intelligence is lesser than the original.
Developer David Horne discussed 1K ZX Chess and published the full source code in a series of articles in Your Computer in 1982 and 1983.
Source: Wikipedia, "[en.wikipedia.org/wiki/1K_ZX_Chess 1K ZX Chess]", available under the CC-BY-SA License.